1. Technical Field of the Invention
The present invention relates to an electronic control apparatus for vehicles, and in particular, to the electronic control apparatus provided with plural computers for electronically controlling electronic devices mounted in vehicles.
2. Related Art
In recent years, electronic devices mounted in a vehicle are mostly controlled by in-vehicle electronic control apparatuses. These electronic control apparatuses have been on the increase in their capability, in which it is frequent that they are provided with a plurality of microcomputers that collaborate with each other in performing objective control actions.
In the foregoing electronic control apparatus, it is also possible to employ the scheme that the apparatus takes a waiting state to wait for realization of a pre-given activation condition assigned to the apparatus during the off state of the vehicle ignition switch. In this case, when it is detected that the activation condition has been met, the apparatus is shifted to its normal operation state. It is required for the microcomputer that the waiting mode should consume power less than the normal operation mode, because of, in no small part, saving the power consumption of the battery. This requirement helps the battery avoid deterioration in the engine startability and prevents the battery from running out.
In consideration of such situations, Japanese Patent Laid-open Publication No. 2007-30593 discloses an electronic control apparatus with two microcomputers. In this control apparatus, a power supply voltage is supplied from a power supply IC to the two microcomputers via a common power line. In addition, when the two microcomputers both output sleep allowing signals to the power supply IC, the power supply IC stops outputting the power supply voltage to both microcomputers. Namely, in this configuration, the power supply IC is in charge of waking up and sleeping the two microcomputers at the same time.
In the above apparatus configuration, both microcomputers can be slept only when the sleep allowing signals from the two microcomputers meets a logical AND condition. Hence, there are provided only two states composed of a first state where the two microcomputers are stopped from operating and a second state where the two microcomputers operate. This means that, if one of the two microcomputers is designated as a main microcomputer and the other is designated as a sub-microcomputer, it is impossible to provide a third state where only the main microcomputer operates while the sub-microcomputer stops. Hence, it is impossible to selectively activate only one microcomputer which is necessary depending on vehicle control situations. This frequently results in unnecessary consumption of the power.
In order to obtain the third state, the person skilled in the art may conceive of a configuration where a reset signal is continuously supplied to the sub-microcomputer for stopping the sub-microcomputer, resulting in the operation of only the main microcomputer. However, in this configuration, both of the sub and main microcomputers are powered continuously. Hence, the power is consumed uselessly by circuit components such as a pull-up resistor and a pull-down resistor incorporated in the sub-microcomputer. As a result, it is not sufficient in saving the power consumption. Moreover, in the above conventional apparatus, another difficulty cannot be avoided, which concerns, for example, a malfunction which may occur in the sub-microcomputer. Practically, if such a malfunction occurs, it is probable that the sub-microcomputer cannot output the sleep allowing signal. If this happens, the apparatus itself no longer reduces the power consumption.